The best thing about these fins is that the rubber is soft and very flexible, giving my girl a very comfortable fit. They don't rub or chafe. The only bad thing about these fins is that because they are so soft, they tore. We got two whole seasons of daily swim out of them before they tore though, so if you are reading this review, it might help to know that I think they are well worth it, and we DID buy new ones.
My daughter is 5 1/2 and on a mini swimteam. She uses the zoomers during some parts of the workout, but mostly for fun with her sister who also got a pair.